In assembling and machining operations of production lines, it has been a popular practice to detect of a position and a shape of each of various objects using various kinds of sensors. Generally, a sensor for taking a two-dimensional image (e.g., a visual sensor using a CCD camera) is effective in measuring the general position of an object. To the contrary, a laser sensor is effective in measuring a position and a shape of a local part of the object.
The laser sensor is easy to mount on the arm end of a robot and in the case where the output of the laser sensor is fetched into the robot controller, the detailed position and shape of a local part of the object can be measured accurately in real time by projecting a laser beam onto the object from a very nearby position and to use the measured data in correcting the position of the robot.
Accordingly, the robot system having a laser sensor is very popular in the field of an arc-welding robot, a sealing robot and a measuring robot, etc.
The laser sensors are chiefly divided into two types, a spot-light-projecting type and a slit-light-projecting type. In the laser sensor of either type, a spot light beam or slit light beam has to be projected accurately on a surface of the object. As long as the object is positioned in a fixed position with some degree of precision, it is easy to quickly realize such a status that a spot beam or a slit beam is projected on the object, by previously teaching a nearby position of the fixed position, as an approach position to the robot.
However, the object would be positioned often with no precision or less reliable precision, depending on the application. In such event, the object has to be searched using a spot or slit light beam of the laser sensor as a probe while moving the robot along a suitable taught path, and if the work could not found out, it would be impossible to start the main measuring (e.g., the shape measuring) using the laser sensor.
Since the laser sensor is inherently unsuitable for large-area measuring, it would take time to search the object and subsequently to approach to the object. On some occasions reliable searching itself would be difficult. Even if it could be successful in finding the object, this fact might not be useful directly in correcting the robot position. For example, it would not always easy to discriminate a position and a direction (e.g., the position of a start point of welding line and the direction of extension) specified according to the posture of the object, from the laser sensor output obtained for the object in unknown posture.